An internal combustion engine can be operated at certain torque and speed combinations to achieve peak fuel efficiency. This knowledge is particularly useful in hybrid vehicle applications architected to allow for selection and control of the engine speed and torque combination as an operating point. An internal combustion engine also produces certain by-products (emissions) as a result of its operation. Depending upon the type of engine, included in these emissions are such things as oxides of nitrogen (NOx), carbon monoxide (CO), unburned hydrocarbons (HC), particulate matter (PM) (i.e., soot), sulfur dioxide (SO2) and noise, for example. It is known that operating an internal combustion engine at peak fuel efficient torque and speed combinations may not result in minimal emission generation. In fact, certain emissions may increase disproportionately to the fuel efficiency gains as the torque and speed conditions converge toward combinations associated with optimal fuel efficiency.
An electrically variable transmission (EVT) can be advantageously used in conjunction with an internal combustion engine to provide an efficient parallel hybrid drive arrangement. Various mechanical/electrical split contributions can be effected to enable high-torque, continuously variable speed ratios, electrically dominated launches, regenerative braking, engine off idling, and multi-mode operation. See, for example, the two-mode, compound split, electromechanical transmission shown and described in the U.S. Pat. No. 5,931,757 to Schmidt, where an internal combustion engine and two electric machines (motors/generators) are variously coupled to three interconnected planetary gearsets. Such parallel EVTs enjoy many advantages, such as enabling the engine to run at high efficiency operating conditions. However, as noted above, such high efficiency operating conditions for the engine may in fact be associated with undesirably high engine emissions.
An EVT control establishes a preferred operating point for a preselected powertrain operating parameter in a powertrain system corresponding to a minimum system power loss. System power loss may include other factors not related to actual power loss but effective to bias the minimum power loss away from operating points that are less desirable because of other considerations such as battery use in a hybrid powertrain.